Allow underscores in XML element names (except for leading characters)

Fixes #569

R=rsc, r
CC=golang-dev
https://golang.org/cl/194121

src/pkg/xml/read.go

@@ -149,14 +149,20 @@ func (p *Parser) Unmarshal(val interface{}, start *StartElement) os.Error {
 // to create a valid Go struct name.  It also converts the
 // name to lower case letters.
 func fieldName(original string) string {
+
+	var i int
+	//remove leading underscores
+	for i = 0; i < len(original) && original[i] == '_'; i++ {
+	}
+
 	return strings.Map(
 		func(x int) int {
-			if unicode.IsDigit(x) || unicode.IsLetter(x) {
+			if x == '_' || unicode.IsDigit(x) || unicode.IsLetter(x) {
 				return unicode.ToLower(x)
 			}
 			return -1
 		},
-		original)
+		original[i:])
 }
 
 // Unmarshal a single XML element into val.

src/pkg/xml/xml_test.go

@@ -5,6 +5,7 @@
 package xml
 
 import (
+	"bytes"
 	"io"
 	"os"
 	"reflect"
@@ -212,3 +213,18 @@ func TestSyntax(t *testing.T) {
 		}
 	}
 }
+
+type item struct {
+	Field_a string
+}
+
+func TestIssue569(t *testing.T) {
+	data := `<item><field_a>abcd</field_a></item>`
+	var i item
+	buf := bytes.NewBufferString(data)
+	err := Unmarshal(buf, &i)
+
+	if err != nil || i.Field_a != "abcd" {
+		t.Fatalf("Expecting abcd")
+	}
+}